An MOS device, used as a charge switch, has inherently zero offset voltage and as an amplifier has very high input resistance. In addition, capacitors are easily fabricated in integrated circuit technology. Therefore, one is led to use capacitors rather than resistors as the precision components in an analog-to-digital (A/D) or digital-to-analog (D/A) converter and to use charge rather than current as the working medium. This technique is referred to as charge-redistribution. A plurality of capacitors can be used as the precision elements of an array and have values equal to C, C/2, C/4, C/8, etc. The accuracy of the A/D (A/D will be used solely henceforth, but will be meant to include both A/D and D/A) converter using the charge redistribution technique is mainly determined by the matching of the capacitors in the array.
Experimentally, it has been shown that in fabricating the capacitors utilizing MOS technology, acceptable ratio matching accuracies of up to about 10 bits can be obtained with good yields. However, to achieve accuracy greater than about 10 bits, external means such as laser trimming is required to change the size and value of the capacitors as required which, in turn, will increase the typical yield. However, laser trimming is a very expensive and time consuming procedure.
A method and apparatus which corrects for errors produced in data acquisition systems is disclosed in U.S. Pat. No. 4,399,426. The '426 patent discloses a method and circuit for correcting errors, such as mismatch between binary weighted capacitors and offset, in a charge redistribution, weighted capacitor array analog-to-digital converter. A self-calibrating, self-correcting circuit is comprised of a second binary array of capacitors which adds to the regular charge redistribution capacitor array an error correcting signal to compensate for the mismatch. This error correcting signal is then stored and the other error correcting signals for other capacitors in the regular capacitor array are determined and subsequently stored for later correction of other capacitance mismatch.